Ring rolling process and apparatus for ring rolling

Abstract

A ring rolling process and corresponding apparatus are disclosed. A ring shaped workpiece is provided, the ring shaped workpiece having a principal axis, an inner radial surface, an outer radial surface, a first axial surface and a second axial surface. The workpiece is subjected to radial pressure between a forming roll 150a acting on the outer radial surface and a mandrel roll 152a, 152b acting on the inner radial surface, at a radial roll bite region. A first axial roll 154a and a second axial roll 156a are provided at the first axial surface and the second axial surface respectively, to subject the workpiece to axial pressure. The first and second axial rolls 154a, 156a are provided at an angular position, measured around the workpiece and with respect to the principal axis, within 10 of said radial roll bite region. Multiple circumferential constraint rolls are provided around the outer radial surface or inner radial surface. In order to control the cross sectional shape of the workpiece, the mandrel roll 152a, 152b and/or the forming roll 150a has a projecting portion for contact with the workpiece, the projecting portion having an axial extent which is smaller than the axial height of the workpiece, the mandrel roll and/or forming roll being axially moveable relative to the workpiece.

Claims

1. A ring rolling process comprising: providing a ring shaped workpiece, the ring shaped workpiece having a principal axis, an inner radial surface, an outer radial surface, a first axial surface and a second axial surface; subjecting the workpiece to radial pressure between a forming roll acting on the outer radial surface and a mandrel roll acting on the inner radial surface, at a radial roll bite region, wherein: a first axial roll and a second axial roll are provided at the first axial surface and the second axial surface respectively, to subject the workpiece to axial pressure, the first and second axial rolls being provided at an angular position, measured around the workpiece and with respect to the principal axis, within 10 of said radial roll bite region; and, in order to control the cross sectional shape of the workpiece at least one of the following conditions (i) and (ii) applies: (i) the mandrel roll has a projecting portion for contact with the workpiece, the projecting portion having an axial extent which is smaller than the axial height of the workpiece, the mandrel roll being axially moveable relative to the workpiece during the ring rolling process so that the projecting portion is applied to different axial locations of the workpiece, in order to control the shape applied to the workpiece; and (ii) the forming roll has a projecting portion for contact with the workpiece, the projecting portion having an axial extent which is smaller than the axial height of the workpiece, the forming roll being axially moveable relative to the workpiece during the ring rolling process so that the projecting portion is applied to different axial locations of the workpiece, in order to control the shape applied to the workpiece; and wherein the first and second axial rolls are independently moved to adapt to the changing cross section of the workpiece during forming.

2. The ring rolling process according to claim 1 wherein an axial roll bite region and the radial roll bite region overlap in terms of angular position around the workpiece.

3. The ring rolling process according to claim 1 wherein circumferential constraint rolls are provided to act on the outer radial surface or inner radial surface of the workpiece.

4. The ring rolling process according to claim 3 wherein the circumferential constraint rolls act to control the compressive or tensional hoop stress in the workpiece.

5. The ring rolling process according to claim 3 wherein there are provided more than two circumferential constraint rolls.

6. The ring rolling process according to claim 5 wherein the more than two circumferential constraint rolls are angularly distributed substantially regularly around the workpiece.

7. The ring rolling process according to claim 3 wherein the circumferential constraint rolls have the same shape as the mandrel and/or forming rolls and are similarly axially moveable relative to the workpiece.

8. A ring rolling apparatus for ring rolling a ring shaped workpiece, the ring shaped workpiece having a principal axis, an inner radial surface, an outer radial surface, a first axial surface and a second axial surface, the ring rolling apparatus comprising: a forming roll and a mandrel roll, for subjecting the workpiece to radial pressure between a forming roll acting on the outer radial surface and a mandrel roll acting on the inner radial surface, at a radial roll bite region, and; a first axial roll and a second axial roll, for subjecting the workpiece to axial pressure between the first axial surface and the second axial surface respectively, the first and second axial rolls being provided at an angular position, measured around the workpiece and with respect to the principal axis of the ring shaped workpiece, within 10 of said radial roll bite region, wherein, in order to control the cross sectional shape of the workpiece at least one of the following conditions (i) and (ii) applies: (i) the mandrel roll has a projecting portion for contact with the workpiece, the projecting portion having an axial extent which is smaller than the axial height of the workpiece, the mandrel roll being axially moveable relative to the workpiece, so that the projecting portion is capable of being applied to different axial locations of the workpiece, in order to control the shape applied to the workpiece; and (ii) the forming roll has a projecting portion for contact with the workpiece, the projecting portion having an axial extent which is smaller than the axial height of the workpiece, the forming roll being axially moveable relative to the workpiece, so that the projecting portion is capable of being applied to different axial locations of the workpiece, in order to control the shape applied to the workpiece; and wherein the first and second axial rolls are independently moveable to adapt to the changing cross section of the workpiece during forming.

9. The ring rolling apparatus according to claim 8 wherein an axial roll bite region and the radial roll bite region overlap in terms of angular position around the workpiece.

10. The ring rolling apparatus according to claim 8 wherein circumferential constraint roll are provided to act on the outer radial surface or inner radial surface of the workpiece.

11. The ring rolling apparatus according to claim 10 wherein the circumferential constraint rolls act to control the compressive or tensional hoop stress in the workpiece.

12. The ring rolling apparatus according to claim 10 wherein there are provided more than two circumferential constraint rolls.

13. The ring rolling apparatus according to claim 12 wherein the more than two circumferential constraint rolls are angularly distributed substantially regularly around the workpiece.

14. The ring rolling apparatus according to claim 10 wherein the circumferential constraint rolls have the same shape as the mandrel and/or forming rolls and are similarly axially moveable relative to the workpiece.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1A shows a schematic plan view of a known ring rolling arrangement.

(3) FIG. 1B shows a schematic partial sectional view of the arrangement of FIG. 1A.

(4) FIG. 2 shows a schematic layout of the ring rolling arrangement of Reference 5.

(5) FIG. 3 shows a schematic partial sectional view of the arrangement of rolls at the radial roll bite region of a ring rolling machine according to Reference 9.

(6) FIGS. 4A and 4B show schematic cross sectional views of workpiece geometries used in the assessment of material flow.

(7) FIG. 5 shows illustrations of different flow patterns due to flexible radial rolling processes.

(8) FIG. 6 plots the results of an upper bound approach to determine the flow mode for different ratios of (ordinate) and (abscissa).

(9) FIG. 7 shows the results of prediction of flow patterns via FEM simulation, illustrating the predicted final cross section of the workpiece for different values of .

(10) FIG. 8 illustrates the operating window for forming L-shapes for different values of C and B where A=0.5.

(11) FIG. 9 illustrates A, B and C for FIG. 8.

(12) FIG. 10 shows a schematic perspective view of a ring rolling apparatus according to an embodiment of the invention.

(13) FIG. 11 shows a view similar to FIG. 10 except that a workpiece is shown in the apparatus.

(14) FIG. 12 shows an enlarged view of the working region of the apparatus of FIG. 10.

(15) FIG. 13 shows a view similar to FIG. 11 except that the circumferential constraint rolls act on the inner radial surface of the workpiece.

(16) FIG. 14 shows a schematic cross sectional view of the roll bite region formed by the radial and axial rolls of a reference example, not inside the scope of the invention.

(17) FIG. 15 shows a schematic cross sectional view of the roll bite region formed by the radial and axial rolls of an embodiment of the invention.

(18) FIGS. 16-21 show a complex cross sectional shape formed from an initial ring-shaped workpiece using ring rolling process according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONAL FEATURES OF THE INVENTION

(19) In the preferred embodiments of the present invention, additional degrees of flexibility are provided compared with known radial profile ring rolling techniques. This offers increased material yield and reduced downstream machining costs, without necessarily requiring expensive part-specific tooling. In the studies underpinning this work, a different approach is taken compared with previous experimental studies. Three key flow patterns are classified, these flow patterns observed in the outer and inner profiling of a ring of intended L-shaped cross-section: axial flow and uniform/non-uniform circumferential flow. The axial height ratio of thick to thin sections and the ring aspect ratio are considered to be key factors determining which of these flow patterns occur. The trends in these factors suggests certain limits to the range of final geometries achievable by simple flexible radial profile ring rolling, in view of undesirable non-uniform flow.

(20) Ring rolling is a bulk metal forming process that typically generates large (1-5 m diameter) metal rings for engineering applications such as aerospace, energy conversion and oil and gas extraction industries. The process conventionally creates metal rings with a rectangular cross-section, unless a profiled tool set is generated to suit each application. Thus, in numerous low-volume ring rolling applications when producing a profiled tool is uneconomical a rectangular ring is made and machined to the final geometry. This results in considerable yield lossesthe difference between input material and material in the finished productand additional machining costs. Ideally, with a single set of universal tools, it would be possible to convert rectangular/barrelled metal ring preforms into a wide range of radially profiled rings.

(21) A typical radial-axial ring rolling machine is shown in FIGS. 1A and 1B. A thick-walled ring-shaped workpiece 10 is thinned in the radial roll bite, between a powered forming roll 12 and idly rotating inner mandrel roll 14. Two guide rolls 20, 22 centre and stabilize the ring 10. A second pair of tools, the lower 26 and upper 28 axial rolls, control the axial height of the ring.

(22) The machine set-up of FIGS. 1A and 1B can be used to generate a non-rectangular shaped ring cross-section if part-specific shaped tooling is used. Inner radial profiles require a shaped mandrel, while outer radial profiles require a shaped forming roll and guide rolls.

(23) A comprehensive experimental study into profile ring rolling at University of Manchester Institute of Science and Technology, UK, showed that profile fillingthe extent to which the cross-section of the workpiece is changed by the profiled toolrequires internal axial flow of material from the radial section that is thinned the most into the section that is thinned the least. However, this is not guaranteed to occur [Reference 1]. The study concluded that in some cases adequate profile filling could only be achieved by starting with ring preforms that are initially shaped. Furthermore, in some applications a set of intermediate profiled tools were needed. Similar conclusions were drawn by Marczinksi [Reference 2] in a discussion of industrial practice in the 1980s; and in FEM simulation studies such as Reference 3. The need for intermediate tooling in generating thin-walled rings such as aero engine casings is also emphasised in the context of reducing yield losses in the industry [Reference 5].

(24) The part specific tooling required for profile ring rolling can be prohibitively expensive to develop for low-volume applications. This motivated work into flexible, or incremental, radial profile ring rolling.

(25) An experimental flexible machine to process wax rings was developed at RWTH Aachen, Germany. FIG. 2 shows the schematic layout of this machine, with an inner mandrel 46 that can move axially (vertically) and thus thin sections of the ring 50 incrementally. Because the tool acts on a small section of an otherwise unconstrained ring, there is an even greater range of possible material flow patterns than in conventional profile rolling. An empirical model for material flow was developed by Tiedemann [Reference 5], predicting the geometrical outcome of a simple tool movement. However, crucially this does not seem to have been inverted to a) determine the tool movements required to achieve a certain shape and b) map out the range of shapes that can actually be achieved with this tooling set-up.

(26) Research is ongoing into novel machine set-ups to improve shaping. Three-roll cross-rolling has been investigated at Wuhan University of Technology, China. In this process, a thick-walled ring is formed between an outer forming roll and two outer passive rolls opposite. Good filling of a deep outer radial groove was achieved; the passive rolls appear to enable the internal axial flow required for profile filling by preventing circumferential flow [Reference 6].

(27) Research into cylindrical ring rolling has shown that it is possible to constrain a ring with a solid sleeve around its circumference, allowing only axial material flow (perpendicular to the conventional rolling direction). This method led to improved filling of an inner profile [Reference 7].

(28) The promotion of axial flow has also been investigated at Dresden University of Technology, Germany. In this technique, outer profiles are incrementally created on long, tubular rings [Reference 8]. A small section of tube is thinned radially by a profiled tool, and since circumferential flow is prevented by the rest of the workpiece the material flows axially.

(29) However, none of these methods could be considered flexible: it is necessary to develop specific tooling for each new part. As yet, to the knowledge of the inventors at the time of writing, no solution exists for reliably generating shaped profiles from non-shaped blanks without part specific tooling. The basis for this solution could lie in an understanding of the flow patterns observed in flexible radial ring rolling, allowing us to determine the range of ring geometries that are achievable.

(30) In order to understand the response of a ring workpiece to incremental radial thinning, an experimental study was carried out on a model ring rolling machine at University of Cambridge, UK. The machine was developed to investigate the effect of novel machine set-ups on achievable ring geometries [Reference 9]. The arrangement of the machine at the radial roll bite is shown schematically in FIG. 3. The workpiece 60 is shown in cross section, along with the forming roll 62, a support roll 64 and a mandrel roll 66. The mandrel roll is capable of axial movement, as well as radial movement, in order to provide a stepped-shape inner radial surface to the workpiece during rolling.

(31) In FIG. 3, refers to the proportion of the workpiece ring height H acted on by the mandrel roll. refers to the reduction in thickness of the workpiece compared with the initial workpiece thickness T.

(32) The results from a chosen sub-set of these experiments in which L-shaped profiles were targeted are discussed below. This type of profile resulted in an interesting range of flow patterns, which are summarised further below. It is thought to be representative of some industrially relevant parts such as weld-neck flanges.

(33) The model material plasticine, a proprietary oil-clay mixture, was used for the experiments. It has been widely used in prediction of flow patterns in metalworking since it has a similar stress-strain flow curve to engineering metals (distinct yield, strain rate hardening), see for example Reference 10.

(34) Ring preforms were prepared in a mould; two sizes were developed representative of a thick and thin walled ring, with differing ratio () of axial height (H) to wall thickness (T). These are shown in FIGS. 4A and 4B respectively, the measurements being in mm.

(35) Six experiments were carried out on each size of preform. Each ring was partially indented by the mandrel to approximately 50% () of its original thickness, over 25, 50, or 75% () of its original axial height, on both the outer or inner radial surface. This therefore amounts of outer and inner profiling.

(36) Three main flow patterns were observable within the results: axial flow, non-uniform and uniform circumferential flow, as illustrated in FIG. 5.

(37) FIG. 5a shows the cross-section of a ring that has principally undergone axial material flow. In this experiment on a thick-walled preform, the outer forming roll tool acted over 50% of the ring's initial height (=50%). The ring has mostly grown in height, and hardly at all circumferentially, indicating that axial material flow was dominant. It appears that the bottom section of the ring was sufficiently large that it remained almost rigid; it was not possible for the action of the tool to achieve sufficient hoop stress in this region for circumferential yield.

(38) The second flow pattern, non-uniform circumferential flow, is shown in FIG. 5b. In this, an inner profile was generated with =50%, but on a thin-walled preform (FIG. 4B). The ring appears almost conical, with the upper section growing in circumference, and the lower section less so, leading to a bent cross-section. There must have been sufficient tensile hoop stress developed in the lower section to allow it to be partially stretched and bent, allowing the upper section to flow in the rolling direction (and slightly axially).

(39) Finally, FIG. 5c shows uniform circumferential flow, for an inner profile with =75%. The ring cross-section remains square as originally intended. This seems to be possible because: a) sufficient material is able to flow internally axially from the top to bottom sections, and b) sufficient hoop stress is developed for it to yield circumferentially.

(40) Analytical modelling and simulation has been carried out in order to predict flow patterns in flexible radial ring rolling of the type described above. Predictions into when a particular flow pattern will occur were made by an upper bound approach, and also inferred from a finite element method (FEM) study of inner profiling.

(41) In the upper bound approach an idealised rigid-plastic velocity field was made for each flow pattern. It was assumed that the velocity field requiring the least work input (plastic work, shear at discontinuities, and friction at the rolls) will be indicative of the real flow pattern.

(42) FIG. 6 shows the results of this upper bound approach by plotting the mode with least work for discretized ratios of and .

(43) If the tool acts over a small section of the ring (small ), axial flow is predicted. For large , uniform circumferential flow is predicted. For intermediate values of , the ring height to thickness ratio, , becomes important: thinner walled rings (large ) are predicted to show non-uniform circumferential growth.

(44) A parametric study was made into the effect of varying the ratio for =50%, using a series of 3D FEM simulations. The simulations were carried out in ABAQUS, with the explicit solver. The simulation suggests a transition from axial growth to non-uniform circumferential growth, as shown in FIG. 7. This is broadly consistent with the experimental results and upper bound analysis prediction.

(45) An illustrative evaluation is now made into the range of achievable geometries from a flexible radial ring rolling process as described above. An operating window approach is used, for L-shapes with a final (not initial) geometry ratio, A=0.5, varying B, and Csee FIG. 9 for an explanation of A, B and C and see FIG. 8 for the operating window. A is the axial proportion of the ring that is thinner compared to the final (and not initial) ring height. B is the aspect ratio of the final cross-section, and C is the final thickness ratio (i.e. thick-thin/thick.

(46) There is potential to make use of axial flow by first rolling to the required outer radius and then shaping the ring upwards. This strategy is limited to relatively low aspect ratio rings (B<1.5-2). There is a probable upper limit on the variation in thickness (e.g. C>0.75). For large B, although the non-uniform circumferential flow mode appears to generate rings with unacceptable conicity, it might be possible to make use of this flow pattern by first acting on the surface of the ring that is to be thinned most, and then acting on the bottom section so as to correct for the conical shape. However, this approach is unlikely to achieve high profile filling (C>0.2-0.4), and would require careful control of the order and amount of indentation on each pass.

(47) On the basis of the work reported above, a ring rolling process for making shaped rings with flexible tooling is possible. Such a process can reduce yield losses and downstream machining costs in low-volume applications.

(48) FIG. 10 shows one embodiment of a ring rolling apparatus according to the present invention. The apparatus, designated generally as 100, is shown here without the workpieces 102, 104 shown in FIGS. 11 and 13 which are otherwise identical and so use similar reference numbers where applicable.

(49) Apparatus 100 includes support table 106 on which is mounted ring-shaped support member 108, having a central axis which is located so as to be coincident with the principal axis of the workpiece. Mounted at different angular positions around ring-shaped support member 108 are support tracks 110. Carriages 112 are linearly movable along support tracks 110 via actuators 114 and have at their forward end axially mounted circumferential constraint rolls 116, adapted to press against either the outer radial surface of the workpiece (FIG. 11) or the inner radial surface of the workpiece (FIG. 13). The circumferential constraint rolls 116 are supported only at one end (here at the top end) in order to allow them to be used at the inner radial surface of the workpiece.

(50) The circumferential constraint rolls 116 are angularly distributed around the apparatus with typically angles of not less than 45 and not more than 90 between adjacent circumferential constraint rolls, as subtended at the central axis of the ring-shaped support member 108, with the possible exception that a greater angle may be subtended between the two circumferential constraint rolls located adjacent the roll bite region, described in more detail below, and opposite the roll bite region.

(51) As discussed above, the ring shaped workpiece has a principal axis, an inner radial surface, an outer radial surface, a first axial surface and a second axial surface. These allow an easier description of the features of the apparatus. The apparatus has a forming roll 120 which is mounted for rotation around a vertical axis and which is driveable by a motor (not shown). The apparatus also has a mandrel roll 122, also mounted for rotation around a vertical axis and which may rotate idly or which may also be driven for rotation by a motor (not shown). Together, the forming roll 120 and the mandrel roll 122 subject the workpiece to radial pressure between them, the forming roll acting on the outer radial surface and the mandrel roll acting on the inner radial surface of the workpiece, at a radial roll bite region.

(52) The apparatus also has a first, lower, axial roll 124 and a second, upper, axial roll 126. These are each rotatable about a horizontal axis, parallel to a radial direction of the workpiece. Together, the first axial roll 124 and the second axial roll 126 subject the workpiece to axial pressure between them, with the first axial roll acting on the first axial surface and the second axial rolls acting on the second axial surface. The first and second axial rolls are positioned in register with each other, in terms of radial and circumferential position. Specifically, the first and second axial rolls are provided at an angular position, measured with respect to the principal axis of the ring shaped workpiece, within 10 of the radial roll bite region. More preferably, the interaction between the first and second axial rolls and the workpiece defines an axial roll bite region (i.e. a region of contact between the workpiece and the first and second axial rolls), and the axial roll bite region and the radial roll bite region overlap in terms of angular position around the workpiece. In this way, the present inventors consider that the flow of the material of the workpiece can be effectively controlled, allowing the development of relatively complex cross sectional shapes. In effect, the arrangement of rolls recreates the mechanics of closed pass radial rolling but in a flexible manner.

(53) FIG. 12 shows an enlarged view of the working region of the apparatus of FIG. 10. The workpiece is absent, to allow the features of the apparatus to be seen. Forming roll 120, mandrel roll 122, first axial roll 124 and second axial roll 126 are shown. It can be seen that the mandrel roll 122 has an annular projection 128 with an axial extent that is typically less than the axial height of the workpiece and less than the axial extent of the forming roll. Additionally, the mandrel roll can be moved axially (as well as radially). During use, therefore, control of the movement of the mandrel roll can be used to develop a specific shape to the inner radial surface of the workpiece, and therefore a specific desired cross sectional shape to the workpiece. Unwanted axial flow of the workpiece during this process is restricted and controlled by the first and second axial rolls.

(54) In an alternative embodiment (not shown), the mandrel roll has a cylindrical shape and an axial extent which is at least as large (and preferably larger) than the axial height of the workpiece. In this embodiment, the forming roll has an annular projection with an axial extent that is less than the axial height of the workpiece and less than the axial extent of the mandrel roll. Additionally, the forming roll can be moved axially (as well as radially). During use, therefore, control of the movement of the forming roll can be used to develop a specific shape to the outer radial surface of the workpiece, and therefore a specific desired cross sectional shape to the workpiece. Unwanted axial flow of the workpiece during this process is restricted and controlled by the first and second axial rolls, as in the embodiment described above. In this embodiment, in which the outer radial surface of the workpiece is profiled and the circumferential constraint rolls bear against the outer radial surface, preferably the circumferential constraint rolls also are axially moveable, in register with the forming roll, and the circumferential constraint rolls have a similar profile shape to the forming roll.

(55) In a further alternative embodiment (not shown), both the mandrel roll and the forming roll have annular projections of the type described above, and both are moveable axially. This allows the development of specific shapes to the inner and outer radial surfaces, further increasing the flexibility of the apparatus to develop complex cross sectional ring shapes. In this case, the circumferential constraint rolls preferably have the form described in the preceding paragraph, with a shape and axial movement matched to the mandrel roll if the circumferential constraint rolls bear against the inner radial surface or matched to the forming roll if the circumferential constraint rolls bear against the outer radial surface of the workpiece.

(56) As explained in the preliminary modelling and experimental work reported above, hoop stress in the workpiece is considered to play an important role in the development of suitable complex geometries in flexible radial ring rolling. Accordingly, the circumferential constraint rolls 116 are deployed in order to control the hoop stress, and in addition stabilise and centralise the workpiece during operation of the apparatus. Using at least three circumferential constraint rolls is expected to assist in the control of the hoop stress, and the inventors consider that use of up to seven circumferential constraint rolls would provide greater control of the hoop stress and this more control over the development of the required cross sectional shape.

(57) It should be noted that FIG. 11 has the circumferential constraint rolls 116 acting on the outer radial surface, therefore promoting compressive hoop stress. In contrast, FIG. 13 has the circumferential constraint rolls 116 acting on the inner radial surface, therefore promoting tensile hoop stress.

(58) The present inventors consider that the provision of the circumferential constraint rolls in the embodiment described above is of interest also in ring rolling techniques where the mandrel roll and the forming roll are each plain cylindrical rolls. Therefore, in a further alternative embodiment (not shown), the circumferential constraint rolls are used in conjunction with cylindrical mandrel and forming rolls and with the first and second axial rolls at the radial roll bite as described above. The additional control over compressive or tensional hoop stress further enhances the control over the material flow at the roll bite. It is noted that Reference 11 uses a large metal sleeve to completely prevent circumferential flow. However, this is inflexible, requiring a new sleeve for each part.

(59) FIG. 14 shows a schematic cross sectional view of the roll bite region formed by the radial and axial rolls of a reference arrangement, which is not inside the scope of the present invention. Here, the forming roll 150 and the mandrel roll 152 have a plain cylindrical shape and rotate about vertical axes. The first (lower) axial roll 154 and the second (upper) axial roll 156 also have a plain cylindrical shape and rotate about horizontal axes. The forming roll 150 and the mandrel roll 152 are independently moveable along their axes of rotation (i.e. to translate up and down) in addition to being rotatable and independently moveable radially. Similarly, the first and second axial rolls 154, 156 are moveable, either independently or together with the mandrel roll and/or forming roll, along their axes of rotation (i.e. to translate radially) in addition to being rotatable and independently moveable vertically. Cooperation of the translation of the rolls 150, 152, 154 and 156 allows them to fit together as shown at the roll bite region, in order to adapt to the changing cross section of the workpiece during forming, but the resultant cross sectional shape of the workpiece being limited to a rectangular cross sectional shape.

(60) FIG. 15 shows a schematic cross sectional view of the roll bite region formed by the radial and axial rolls of an embodiment of the invention. This is a modification of FIG. 14, the modification here being provided at the mandrel roll. Here, the forming roll 150a has a plain cylindrical shape and rotates about a vertical axis. The first (lower) axial roll 154a and the second (upper) axial roll 156a also have a plain cylindrical shape and rotate about horizontal axes. It is preferred that the forming roll 150a is moveable along its axis of rotation (i.e. to translate up and down) in addition to being rotatable and independently moveable radially. Similarly, the first and second axial rolls 154a, 156a are independently moveable along their axes of rotation (i.e. to translate radially) in addition to being rotatable and independently moveable vertically. Mandrel roll 152a rotates about a vertical axis and is moveable along its axis of rotation (i.e. to translate up and down) in addition to being rotatable and independently moveable radially. Mandrel roll 152a has an annular projection 152b that is relatively narrow in axial extent compared with, for example, the forming roll. Control of the translation of the mandrel roll therefore allows the development of relatively complex shapes for the inner radial surface of the workpiece and correspondingly complex cross sectional shapes for the workpiece.

(61) FIGS. 16-21 show a complex cross sectional shape formed from an initial ring-shaped workpiece using ring rolling process according to an embodiment of the invention.

(62) FIG. 16 shows the workpiece 202 in cross section parallel to its axis of rotation in an apparatus according to an embodiment of the invention (shown schematically) at the beginning of the ring rolling process. FIG. 17 shows the workpiece at substantially the same stage of the process, in a cross section perpendicular to its axis of rotation.

(63) Mandrel roll 252 bears against the radially inner surface of the workpiece 202. Mandrel roll 252 has an axial height sufficient to contact the entire axial height of the workpiece. In this embodiment, the mandrel roll is not moved axially, only radially in order to ensure the increase in radius of the workpiece during the process. Forming roll 250b has an axial height which is less than the starting axial height of the workpiece. The effect of this is that forming roll 250b makes contact with only part of the outer radial surface of the workpiece during a revolution of the workpiece. In FIG. 16, the forming roll 250b makes contact with the upper part of the outer radial surface of the workpiece.

(64) Upper 256 and lower 254 axial rolls make contact with the upper and lower axial surfaces of the workpiece 202, respectively.

(65) Six circumferential constraint rolls 216 are provided, as shown in FIG. 17. These control and maintain compressive hoop stress in the workpiece during the process, as described above.

(66) The effect of the forming roll 250b bearing against only the upper part of the outer radial surface of the workpiece 202 is that a step-shaped profile is developed in the outer radial surface of the workpiece 202.

(67) FIGS. 18 and 19 show a later stage in the same ring rolling process. Similarly to FIGS. 16 and 17, FIG. 18 shows the workpiece 202 in cross section parallel to its axis of rotation at an intermediate time during the ring rolling process. FIG. 19 shows the workpiece at substantially the same stage of the process, in a cross section perpendicular to its axis of rotation.

(68) In FIGS. 18 and 19, the step-shaped profile of the workpiece has been developed to a substantial degree. Subsequently, the forming roll 250b has been moved axially relative to the workpiece and relative to the mandrel roll 252, to bear against the remaining part of the outer radial surface of the workpiece.

(69) The shape of the finished workpiece is shown in FIG. 20 (cross section parallel to the axis of rotation) and FIG. 21 (cross section perpendicular to the axis of rotation). As a result of the part of the process shown in FIGS. 18 and 19, the diameter of the workpiece has been enlarged further compared with FIGS. 18 and 19, and the depth of the step at the outer radial surface has been reduced due to the work carried out on the lower part of the outer radial surface of the workpiece, with the material flow guided and constrained by the mandrel roll and the upper and lower axial rolls, and the circumferential constraint rolls 216 providing stabilising compressive hoop stress and bearing against the same axial part of the outer radial surface of the workpiece as the forming roll 250b.

(70) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(71) All references referred to above and/or listed below are hereby incorporated by reference.

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